Engine systems having internal combustion engines are frequently provided with supercharging devices such as exhaust gas-driven turbochargers for the purpose of supplying the internal combustion engine with fresh air under increased pressure. For this purpose, the supercharging devices include compressors which are connected via a shaft to a turbine, which is driven by an exhaust gas flow from the internal combustion engine.
In engine systems having supercharging devices, a so-called compressor stall at the compressor may occur in an air supply system if the load changes rapidly from a high load to a low load. The compressor stall produces an irritating noise and may cause damage to the impellers of the compressor. To avoid this, a blowoff line is usually provided with a blowoff valve which connects an output side of the compressor to its input side. The blowoff valve is closed during normal operation and opened only in operating states in which the compressor could otherwise enter an unstable operating state in which a compressor stall occurs.
To detect faults at an early stage, the blowoff valve may be regularly subjected to an on-board diagnosis in which its function is checked.
The diagnosis of the blowoff valve ordinarily takes place by evaluating the intake air mass flow or the boost pressure on the output side of the compressor. In both variables, the compressor stall is easily identified by a corresponding oscillation.
In modern engine systems, it is possible to dispense with the use of a mass air flow sensor on the input side of the air supply system, so that a diagnosis by evaluating the corresponding sensor signal is not possible. Moreover, engine systems are known which have two consecutively situated supercharging devices or two supercharging devices situated in parallel, so that a compressor stall in the individual compressors is no longer detectable by evaluating the boost pressure variable, since the pressure waves of the two compressors may, under some circumstances, overlay each other and thus temporarily extinguish each other.